The present invention generally relates to electronic equipment, and more particularly to a system that utilizes passive eddy current speed/rotation sensors to sense rotating equipment, such as blades of a gas turbine.
Passive eddy current sensors (also known as variable reluctance sensors) have been employed in a wide variety of applications to sense the proximity and speed of rotating equipment, including blades (buckets) of gas turbines. The clearance between turbine blade tips and the shroud surrounding them varies depending on the temperatures of the blades and shroud, with greater clearances occurring at start-up or otherwise when the engine is cooler. Improved control of a gas turbine engine can be achieved by monitoring the clearance between the shroud and blade tips, as well as the rotational speed of the blades. The output of a passive eddy current sensor (or other suitable proximity sensor) positioned to sense blade rotation and blade-shroud clearance can be used to modify the engine operation and/or, if so equipped, control the shroud to maintain a desired blade-shroud clearance.
Passive eddy current sensors typically contain one or more permanent magnets adjacent one or more ferromagnetic cores wound with a wire coil. The permanent magnet is typically formed of a high magnetic energy product material, such as iron-rare earth metal alloys (for example, Nd—Fe—B) and samarium alloys (for example, Sm—Co). The core is typically formed of a magnetic material, such as a magnetic steel, though other suitable magnetic materials including low carbon steels may be used depending on operating conditions. When used to monitor the clearance between a shroud and blade tips, a passive eddy current sensor is mounted to maximize the electrical signal generated as each blades passes in proximity to the sensor. In particular, the sensor is oriented so that, in the absence of a blade, magnetic flux is directed through one end of the magnet and toward the turbine rotor and its blades, then arcs back through space to the ferromagnetic core. When a blade passes through the magnetic field, eddy currents form in the blade material and the local magnetic field shifts, producing a voltage potential across the leads of the coil. Because engine casings are typically formed largely of titanium, nickel, and other nonferrous materials that exhibit low magnetic reluctance, the ends or the magnet and core are not required to be inserted entirely through the engine casing, but instead can be mounted in an external recess in the wall such that a portion of the wall separates the sensor for the hot gas path of the engine.
In modern gas turbine engines, the output of a passive eddy current sensor used to monitor blade rotation and/or blade-shroud clearance is delivered to the engine's FADEC (full authority digital engine control) through appropriate connectors and wiring. To some degree, passive eddy current sensors and any electronics associated therewith (for example, signal processing circuitry) are subjected to the harsh environment of the gas turbine, including high temperatures and electromagnetic radiation. Electronics are typically limited to about 125° C., and consequently must be cooled if located in proximity to a passive eddy current sensor. U.S. Pat. No. 7,170,284 to Roeseler et al. is an example. In addition, passive eddy current sensors are susceptible to electromagnetic interference (EMI) noise due to the many turns of wire typically present and required in the construction of their cores. Testing of passive eddy current sensors employing multiple coils with a simple wire connection between coils has shown that the combined resistance and inductance of the wire and coils are too great for the sensor to have sufficient bandwidth to accurately perform the desired sensing task.